The present invention relates to a semiconductor laser having an active layer buried in a groove.
FIG. 1 shows, in cross section, an example of a conventional semiconductor laser of this type. In FIG. 1, reference numeral 1 depicts an n type InP substrate, 2 an n type InP buffer layer, 3 a p type InP current blocking layer, 4 an n type InP current blocking layer, 5 an n type InP cladding layer, 6 an InGaAsP active layer, 7 a p type InP cladding layer, 8 a p type InGaAsP contact layer, 9 a p side electrode, and 10 an n side electrode.
The n type InP and p type InP current blocking layers 4 and 3 are arranged on opposite sides of the InGaAsP active layer 6, the latter having a width of about 2 microns. The laser becomes operative when a forward bias voltage is applied across the p side electrode 9 and the n side electrode 10. Since, in this case, a boundary region between the n type InP current blocking layer 4 and the p type InP current blocking layer 3 becomes a p-n reversely biased junction, the injection current to the laser is concentrated in the InGaAsP active layer 6 in a groove 11; the amount of leakage current flowing through regions other than the active layer 6 is minor. As a result, it is possible to obtain oscillation with a threshold current value of 20 mA or less. Further, since the thickness of the InGaAsP active layer 6, measured at the center of the crescent-shaped region in FIG. 1, is on the order of 0.1 to 0.15 microns, and the InGaAsP active layer 6 is surrounded by InP crystals of a refractive index lower than that of the active layer 6, a waveguide is formed integrally therewith, whereby a stable fundamental transverse mode operation is realized.
In the conventional semiconductor laser described above, however, atoms of crystals grown on the flat portion around the groove 11 tend to diffuse towards the interior of the groove 11 when the n type InP cladding layer 5, the InGaAsP active layer 6, and the p type InP cladding layer 7, etc., are grown by liquid phase epitaxy. Therefore, the crystal growth speed is generally higher in the groove than on the flat portion, and thus it becomes difficult to control the thickness and position of the InGaAsP active layer in the groove, thereby rendering the reproducibility of crystal growth poor.